Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755658AbXHQTez (ORCPT ); Fri, 17 Aug 2007 15:34:55 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750897AbXHQTet (ORCPT ); Fri, 17 Aug 2007 15:34:49 -0400 Received: from x346.tv-sign.ru ([89.108.83.215]:51074 "EHLO mail.screens.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750805AbXHQTes (ORCPT ); Fri, 17 Aug 2007 15:34:48 -0400 Date: Fri, 17 Aug 2007 23:34:53 +0400 From: Oleg Nesterov To: Andrew Morton , David Howells , "David S. Miller" , Nick Piggin , "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org Subject: [PATCH,RESEND] documentation: atomic_add_unless() doesn't imply mb() on failure Message-ID: <20070817193453.GA198@tv-sign.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.11 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1968 Lines: 46 (the explicit ack/nack from maintainers is wanted). A "typical" implementation of atomic_add_unless() can return 0 immediately after the first atomic_read() (before doing cmpxchg). In that case it doesn't provide any barrier semantics. See include/asm-ia64/atomic.h as an example. We should either change the implementation, or fix the docs. Signed-off-by: Oleg Nesterov Documentation/atomic_ops.txt | 3 ++- Documentation/memory-barriers.txt | 2 +- 2 files changed, 3 insertions(+), 2 deletions(-) diff -puN Documentation/atomic_ops.txt~doc-atomic_add_unless-doesnt-imply-mb-on-failure Documentation/atomic_ops.txt --- a/Documentation/atomic_ops.txt~doc-atomic_add_unless-doesnt-imply-mb-on-failure +++ a/Documentation/atomic_ops.txt @@ -137,7 +137,8 @@ If the atomic value v is not equal to u, returns non zero. If v is equal to u then it returns zero. This is done as an atomic operation. -atomic_add_unless requires explicit memory barriers around the operation. +atomic_add_unless requires explicit memory barriers around the operation +unless it fails (returns 0). atomic_inc_not_zero, equivalent to atomic_add_unless(v, 1, 0) diff -puN Documentation/memory-barriers.txt~doc-atomic_add_unless-doesnt-imply-mb-on-failure Documentation/memory-barriers.txt --- a/Documentation/memory-barriers.txt~doc-atomic_add_unless-doesnt-imply-mb-on-failure +++ a/Documentation/memory-barriers.txt @@ -1492,7 +1492,7 @@ about the state (old or new) implies an atomic_dec_and_test(); atomic_sub_and_test(); atomic_add_negative(); - atomic_add_unless(); + atomic_add_unless(); /* when succeeds (returns 1) */ test_and_set_bit(); test_and_clear_bit(); test_and_change_bit(); _ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/